Blender 2.8 Lag issue on grease pencil with multiple layers. Possible temporary solution
I know what your saying, I run into a tonne of lag when animating grease pencil in full scenes. My favourite way so far to get around this is to finish of the main parts of the scene that effect your character and view (e.g Camera animation and interactive objects, then save the scene then strip the scene right down and save it again using a file name that is relevant to the file (GP animation etc.) then do your grease pencil work. Once finished save and go back to you main file create an empty and then "Proxy" the GP collection into your scene and append it to your empty. Move the empty to the desired position and render. Hope this helps